The present invention relates to a method and apparatus for the non-invasive determination of the cardiac output of a subject, and particularly to a method and apparatus which utilizes a gas analysis of the air flow through a respiratory device to determine cardiac output, as well as other physiological conditions of a subject, such as oxygen consumption, carbon dioxide production, etc.
U.S. Pat. Nos. 5,038,792; 5,178,155 and 5,179,958, all by the same inventor as the present application, relate to systems for measuring metabolism and related respiratory parameters, such as oxygen consumption and carbon dioxide production through indirect calorimetry using a respiratory gas analyzer device measuring metabolic activity of the subject. My patent application Ser. No. 814,677, filed Mar. 11, 1997, now U.S. Pat. No. 5,836,300, discloses such a respiratory gas analyzer for measuring, not only the metabolic activity of a subject, but also the cardiac output of the subject in a non-invasive manner. The systems described in the above patents employ bi-directional flow meters which pass both the inhalations and the exhalations of the subject breathing through the device, and integrate the resulting instantaneous flow signals to determine total full flow volumes. The concentration of carbon dioxide generated by the subject may be determined by passing the exhaled volume through a carbon dioxide scrubber before it is passed through the flow meter so that the difference between the inhaled and exhaled volumes is essentially a measurement of the carbon dioxide contributed by the lungs. Alternatively, the concentration of carbon dioxide may be determined by measuring the instantaneous carbon dioxide content of the exhaled volume with a capnometer, and integrating that signal with the exhaled flow volume. The oxygen consumption can then be calculated by subtracting the carbon dioxide content from the exhaled volume, and then subtracting the resulting exhaled volume from the inhaled volume.
The systems described in the above-cited patents generally use a scrubber for removing the carbon dioxide in order to permit a determination to be made of the carbon dioxide content of the air, particularly the exhaled air. Such scrubbers, however, are relatively bulky and require replenishment after extended use. In addition, some of the described systems required capnometers for measuring the carbon dioxide concentration. Such capnometers have to be highly precise, and are therefore very expensive, because any errors in measurement of the carbon dioxide content of the exhalations produces a substantially higher error in the resulting determination of the oxygen content, or the carbon dioxide content, of the exhalation.
One object of the present invention is to provide a method and apparatus which enable the use of such a respiratory gas analyzer for the non-invasive determination of the cardiac output of a subject, as well as for the measurement of oxygen consumption and/or carbon dioxide production.
Another object of the present invention is to provide a method and apparatus for the non-invasive determination of the cardiac output of a subject, which method and apparatus do not require a scrubber for removing the carbon dioxide from the air volume.
A further object of the invention is to provide such a method and apparatus which do not require a capnometer for sensing the carbon dioxide content, but which could include such a capnometer in order to improve accuracy.
A still further object of the invention is to provide such a method and apparatus which may also be used for determining oxygen consumption, and/or carbon dioxide production, as well as other metabolic or cardiovascular conditions.
According to one aspect of the present invention, there is provided a method for the non-invasive determination of the cardiac output of a subject by:
(a) causing the subject to inhale and exhale air via a respiratory tube in a plurality of breathing cycles including normal breathing cycles in which the inhaled air does not receive any significant amount of exhaled air from the preceding cycle, and rebreathing cycles in which the inhaled air receives an end tidal portion of the exhaled air from the preceding cycle;
(b) measuring the carbon dioxide content in the exhaled air during both the normal breathing cycles and the rebreathing cycles; and
(c) utilizing the carbon dioxide content measurements to determine the cardiac output of the subject.
According to further features in this preferred embodiment of the invention described below, the carbon dioxide content is measured without the use of a scrubber by:
(1) measuring the carbon dioxide concentration in the exhaled air during both the normal breathing cycles and the rebreathing cycles;
(2) propagating ultrasonic pulses obliquely through the air passing through the respiratory tube;
(3) measuring the transit times of said pulses;
(4) computing from the measured transit times the flow volume; and
(5) multiplying the flow volume by the measured carbon dioxide concentration.
According to still her features in the preferred embodiment of the invention described below, the carbon dioxide concentration is measured by computing from the measured transit times the fraction of carbon dioxide in the exhaled air. More particularly, the carbon dioxide concentration in the exhaled air is computed by:
(i) determining from the measured transit times the oxygen fraction in the inhaled air (FIO2) and in the exhaled air (FEO2); and
(ii) computing the carbon dioxide content (VCO2) in the exhaled air according to the following equation:
xe2x80x83VCO2=[VExe2x88x92(VExc2x7FEO2)]xe2x88x92[VIxe2x88x92(VIxc2x7FIO2)]
wherein VE and VI are the measured volumes of the inhaled air and exhaled air, respectively.
Thus, the constituents of the exhaled gas, other than nitrogen, oxygen and carbon dioxide, may be ignored. Since carbon dioxide has a substantially higher density than oxygen, and moles of oxygen and carbon dioxide occupy the same volume, it will be seen that the instantaneous carbon dioxide content of the exhaled air may be calculated with a reasonable degree of accuracy simply from the measurements of the mass of the inhaled and exhaled gases. The exhaled O2 concentration [O2]e and the exhaled CO2 concentration [CO2]e are calculated from the exhaled mass and volume, and, knowing the inhaled O2 concentration [O2]i, the oxygen volume [VO2] is then calculated by the following equation:       VO    2    =                    I        -                              [                          O              2                        ]                    e                -                              [                          CO              2                        ]                    e                            1        -                              [                          O              2                        ]                    i                      xc3x97          (                                    [                          O              2                        ]                    i                -                              [                          O              2                        ]                    e                    )        ⁢    Vek  
where k is a non-adiabatic correction constant to compensate for the non-ideal nature of the cases.
The CO2 volume (VCO2) is calculated as:
VCO2=[CO2]exVe
Where Ve is the total exhaled volume.
An ultrasonic flow meter, such as described in U.S. Pat. Nos. 4,425,805; 4,914,959 or 5,645,0791, may be used for this purpose. The use of an ultrasonic transit time flow meter for measuring the carbon dioxide content of the exhaled gas avoids the need of a scrubber. It also avoids the need of a capnometer for measuring carbon dioxide concentration, and an oxygen sensor, operating upon the respiratory gasses as they pass through the flow tube, and thereby enables the gasses to pass in a substantially continuous and uninterrupted manner to provide high uniformity in the measurement.
While the preferred embodiment described below utilizes the ultrasonic flow meter to measure flow volume, as well as carbon dioxide and/or oxygen concentration, the described technique may also use a conventional capnometer to sense the carbon dioxide content of the exhaled air, and/or an oxygen sensor for sensing the oxygen content of the exhaled air. The use of an oxygen sensor as an alternative to a capnometer provides the advantages of lower cost, higher reliability, and higher accuracy of the oxygen measurement. It is also contemplated that another type of flow meter may be used, other than the ultrasonic flow meter, but with the carbon dioxide sensor and/or the oxygen sensor.
The described technique may be used not only for non-invasively determining cardiac output, but also for non-invasively determining total oxygen consumption, carbon dioxide production, and other metabolic and/or cardiovascular conditions of the subject by merely analyzing the respiratory gasses produced during breathing in accordance with the techniques described in my above-cited patents.
Further features and advantages of the invention will be apparent from the description below.